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Kamakura, Japan

Takeshita S.,Keio University | Isobe T.,Keio University | Sawayama T.,SINLOIHI Co. | Niikura S.,SINLOIHI Co.
Progress in Crystal Growth and Characterization of Materials | Year: 2011

YVO4:Bi3+,Eu3+ nanophosphors are prepared by low-temperature wet chemical precipitations in the presence of citrate ions. Transparently dispersed YVO4:Bi3+,Eu3+ nanophosphors crystallize from amorphous gel precursors through a gel-sol process. The homogeneity of Bi3+ doping process as well as the photoluminescence property are improved by using the homogeneous Bi3+ solution as a Bi3+ source instead of Bi3+ powder. YVO4:Bi3+,Eu3+ nanophosphor under the irradiation of near-UV light shows peculiar photobleaching behavior through the reduction from V5+ to V4+, but this photobleach is suppressed by subsequent hydrothermal treatment. The authors also discuss the advantage of transparency of YVO4:Bi3+,Eu3+ nanophosphor in terms of wavelength conversion as compared to micron-sized phosphor. © 2011 Elsevier Ltd. All rights reserved.

Hara H.,Keio University | Takeshita S.,Keio University | Isobe T.,Keio University | Nanai Y.,University of Electro - Communications | And 3 more authors.
Journal of Alloys and Compounds | Year: 2013

We report a glycothermal synthesis of Ce3+-doped YBO3 mesocrystals and their Ce3+-concentration dependence of particulate and photoluminescent properties. Submicrometer-sized YBO3:Ce 3+ disk-like particles with Ce3+ concentrations of 0-3 at.% were formed from trimethyl borate and acetates of yttrium and cerium(III) via a glycothermal reaction at 300 °C for 2 h in 1, 4-butanediol. The size of the particles decreased from ∼1.5 μm to ∼150 nm with increasing the Ce3+ concentration up to 1 at.%. Selected area electron diffraction revealed that each disk-like particle was a mesocrystal, i.e., an assembly of nanocrystals with a uniform crystallographic orientation, irrespective of the Ce3+ concentration. The particles showed blue emission through 5d-4f transitions of Ce3+ under near-UV excitation. Concentration quenching of the Ce3+ emission for the glycothermally-prepared YBO3:Ce3+ started from a lower Ce3+ concentration than that of solid-state-prepared bulk YBO 3:Ce3+, indicating an inhomogeneous distribution of Ce3+ ions in the mesocrystals. © 2013 Elsevier B.V. All rights reserved.

Sato R.,Keio University | Takeshita S.,Keio University | Isobe T.,Keio University | Sawayama T.,SINLOIHI Co. | Niikura S.,SINLOIHI Co.
ECS Journal of Solid State Science and Technology | Year: 2012

YBO3:Ce3+,Tb3+ phosphor was prepared from rare earth oxides and boric acid by a solid state reaction. The phosphor absorbs near UV light through 4f-5d transitions of Ce3+, followed by broad emissions through 5d-4f transitions of Ce3+ and sharp emissions through 4f-4f transitions of Tb3+. Spectroscopic investigations for samples with various Ce3+ and Tb3+ concentrations reveal nonradiative energy transfer from Ce3+ to Tb3+. Emission color of the YBO3:Ce3+,Tb3+ varies from blue (0.163, 0.019) to green (0.321, 0.585) depending on the Ce3+ and Tb3+ concentrations. The optimized green-emitting Y 0.82Ce0.03Tb0.15BO3 phosphor has an emission color of (0.309, 0.547) with an external quantum efficiency of 76.7%. The photoluminescence intensity of this phosphor at 150°C keeps 87% of its intensity at room temperature, showing sufficient thermal stability for white light emitting diode applications. © 2012 The Electrochemical Society.

Takeshita S.,Keio University | Honda J.,Keio University | Isobe T.,Keio University | Sawayama T.,SINLOIHI Co. | Niikura S.,SINLOIHI Co.
Journal of Solid State Chemistry | Year: 2012

The influence of aging of the suspension containing the amorphous precusors on structural, compositional and photoluminescent properties is studied to understand the mechanism on the formation of Zn 2GeO 4:Mn 2 nanoparticles during the solvothermal reaction in the water/diethylene glycol mixed solvent. Aging at 200 °C for 20 min forms the crystalline Zn 2GeO 4 nanorods and then they grow up to ∼ 50 nm in mean length after aging for 240 min. Their interplanar spacing of (410) increases with increasing the aging time. The photoluminescence intensity corresponding to the d-d transition of Mn 2 increases with increasing the aging time up to 120 min, and then decreases after aging for 240 min. The photoluminescence lifetime decreases with increasing the aging time, indicating the locally concentrated Mn 2 ions. These results reveal that Mn 2 ions gradually replace Zn 2 ions near surface through repeating dissolusion and precipitation processes during prolonged aging after the complete crystallization of Zn 2GeO 4. © 2011 Elsevier Inc. All rights reserved.

Takeshita S.,Keio University | Honda J.,Keio University | Isobe T.,Keio University | Sawayama T.,SINLOIHI Co. | Niikura S.,SINLOIHI Co.
Crystal Growth and Design | Year: 2010

Zn2GeO4:Mn2+ nanophosphors are synthesized from germanium(IV) oxide and acetates of zinc and manganese(II) in a mixed solvent of water and diethylene glycol (DEG) by the solvothermal reaction at 200 °C for 2 h. Phase pure Zn2GeO4:Mn2+ is obtained for the samples prepared at 0 ≥ xDEG ≥ 91.7, where xDEG is the volume percentage of DEG in the mixed solvent. The particle and crystallite sizes decrease with the increase in xDEG. The sample prepared at xDEG = 91.7 comprises the nanorods with 30.2 and 12.2 nm in mean length and width, respectively. The actual Mn concentration measured by X-ray fluorescence analysis increases with the increase in x DEG and approaches to the nominal concentration, 2.0 at %. The samples show green luminescence corresponding to the d-d transition of Mn 2+ under the irradiation of UV and near-UV light. The photoluminescence intensity reaches a maximum around xDEG ∼ 80. This is attributed to the competitive factors, the Mn concentration and the sizes of crystallites and particles. © 2010 American Chemical Society.

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